A hydraulic drive system for use in hydraulic machines such as hydraulic excavators comprises a hydraulic pump, a plurality of hydraulic actuators driven by a hydraulic fluid supplied from the hydraulic pump, and a valve apparatus including a plurality of directional control valves to control respective flow rates of the hydraulic fluid supplied from the hydraulic pump to the plurality of actuators.
In this type hydraulic drive system, load sensing control has been proposed for controlling a delivery pressure of the hydraulic pump in response to the load pressure mainly from the viewpoint of energy saving. Examples of the load sensing control are disclosed in GB 2,195,745A, U.S. Pat. No. 4,425,759, EP 0,366,815A1, etc. In the disclosed prior art, the hydraulic drive system has means for taking out a maximum one of the load pressures of the plural actuators. The plural directional control valves each comprise a supply passage communicating with the hydraulic pump, a load passage communicating with a corresponding one of the actuators, a first passage capable of communicating with the supply passage, a second passage capable of communicating with the first passage and the load passage, a flow control valve for controlling a flow rate of the hydraulic fluid passing between the supply passage and the first passage dependent upon an opening of a variable restrictor positioned therebetween, and also selectively communicating between the first passage and the second passage, and a pressure control valve located between the first passage and the second passage for controlling a pressure inside the first passage. The pressure control valve comprises a valve body having a first pressure receiving sector operative in a valve opening direction and a second pressure receiving sector operative in a valve closing direction, a first control chamber to which the pressure inside the first passage is introduced for causing the introduced pressure to act on the first pressure receiving sector, and a second control chamber to which the maximum load pressure is introduced as a first control pressure for causing the first control pressure to act on the second pressure receiving sector. With such construction of the pressure control valve, the pressure inside the first passage is controlled in response to the maximum load pressure so that a differential pressure across the flow control valve is held at a predetermined value in relation to the load sensing control.
The first and second pressure receiving sectors of the pressure control valve in the above construction are usually, as described in GB 2,195,745A and U.S. Pat. No. 4,425,759, constant in their pressure receiving areas and so is the differential pressure across the flow control valve controlled by the pressure control valve. As a result, flow rate characteristics of the flow control valve cannot be changed. Meanwhile, in the valve body of EP 0,366,815A1, the second pressure receiving sector in the valve closing direction is divided into two central and peripheral pressure receiving sectors, and separate control chambers are provided in association with those two pressure receiving sectors. The maximum load pressure is always introduced to the control chamber associated with the central pressure receiving sector, whereas the maximum load pressure and the reservoir pressure are selectively introduced to the peripheral pressure receiving sector upon a switch valve being actuated. This allows the pressure inside the first passage to be controlled to different values dependent upon whether the maximum load pressure or the reservoir pressure is introduced to the control chamber associated with the peripheral pressure receiving sector. As a result, the differential pressure across the flow control valve is variable to change flow rate characteristics thereof.
However, the prior art described in EP 0,366,815A1 has suffered from the following problem.
First, in the pressure control valve described in EP 0,366,815A1, depending upon whether the maximum load pressure or the reservoir pressure is introduced to the control chamber associated with the peripheral pressure receiving sector, the differential pressure across the flow control valve is variable to change flow rate characteristics thereof as mentioned above. However, the differential pressure across the flow control valve as developed when the reservoir pressure is introduced to the control chamber is, as will be seen from Equation (22) described later, expressed by an equation including the maximum load pressure and thus undergoes an influence of the maximum load pressure. Accordingly, upon change of the maximum load pressure, the differential pressure across the flow control valve is changed and so are the flow rate characteristics thereof. This leads to the problem that the actuator cannot be driven at a desired speed and the operability deteriorates.
The second problem is as follows. In the above prior art, by introducing the reservoir pressure to the control chamber associated with the peripheral pressure receiving sector, the flow rate characteristics can be changed such that the force acting on the valve body in the valve closing direction is reduced to increase the differential pressure across the flow control valve. It is however impossible to decrease the differential pressure across the flow control valve. Accordingly, the flow rate characteristics cannot be varied to lessen the flow rate passing through the flow control valve, meaning that the flow control valve cannot have flow rate characteristics suitable for those works which require fine operation of the actuator as encountered in horizontal drawing of a bucket and fine control of the entire machine.